A definite "No" from me! I have no problem with parents bathing with young children, but a grandad? Why? My husband wouldn't dream of suggesting he bathed with our grandchildren, and neither would I. It's so unnecessary, and it just doesn't seem right. As others have said, it's difficult to actually say anything to the parents if they are OK with the situation, and you certainly wouldn't want to suggest any wrong doing, but I'm afraid if it were me, now the children are getting older, I'd have to say something to my son. Even if it is all purely innocent, imagine how the parents would feel if one of the kids mentioned something to a teacher, and they were called into school to discuss it! Even though your DiL has been brought up in a liberal minded family, it surprises me that your son is OK with this.